The invention relates to a method of controlling the fixing temperature of a powder image in an electrophotographic copying machine.
It is well known in the art of electrophotography that a powder or toner image obtained on a copy sheet can be fixed as a permanent image by causing the toner to be melted under heat to be fused with the copy sheet. A heating element may be used to this end and directly fed from a power source. A fixing unit of this kind is subject to a large variation in the amount of power supplied thereto in response to a fluctuation in the source voltage. Since the power supplied to the heating element is proportional to the square of the source voltage, it can be seen that the power may vary as much as 50% between source voltages of 90 and 100 volts, for example, as shown below. EQU 110.sup.2 /90.sup.2 =12100/8100.perspectiveto.1.5
This results in a large change in the fixing temperature, and causes a substantial degradation in the fixing characteristic of the powder image to the sheet.